Method for stabilizing semi-conductor rectifiers



Feb. 24; 1959 w. F. HALDE'MAN METHOD FOR STABILIZING SEMI-CONDUCTORRECTIFIERS Filed Feb. 15. 1953 FIG 2 FIG ROOM TEMP .OOI AMPS FORWARDCURRENT 4 @320 n moadh mwm OmdBmOu CYCLEQ F ORWARD G URRENT, I-MILLIAMPERS HOT- COLD FIG 3 F ORWARD CURRENT, 1""ILLIAMPERS INVENTOR.WILLIAM F. HALDEMAN nited States Pa 'fifo r 2,874,448 NIETHOD FORSTABILIZING SEMI-CONDUCTOR RECTIFIERS William'F. I Ialdeman,Indianapolis, Ind. I Application February 13, 1953, Serial No. 536,892 9Claims. c1. 29-253 (Granted under Title 35, U.;s. Code (1952 sec. 266)The invention described herein may be manufactured and used by or forthe Government of the United States of America for governmental purposeswithout the payment of any royalties thereon or therefor.

.My invention relates to semiconductor-type rectifiers andis,particularly directed to methods and means for stabilizing theresistance-current characteristics of said rectifiers. p

It has long been known that two dissimilar metals placed in contact haveunidirectional current carrying characteristics and. in recent years themetal germanium has beenextensively used as one of these metals, the

second metal being a cat whisker made of steel ma nonferrous metalspring-pressed into contact with the germanium. In an effort toaccentuate the unidirectionality of such a rectifier and to standardizethe current-resistance characteristics, the germanium is melted with afraction ofa percentage of an impurity and' permitted to cool slowly. Inspite of care in preparing the germanium ingot there is found to beradical variations in electrical characteristics among different ingotsand among difierent parts of each ingot. Worse, the germanium changesits characteristics with aging, thus upsetting circuitry in which ithasbeen incorporated and calibrated. I I have found that commercialsemiconductor-type rectifiers have resistance-current characteristicsthat may be defined by the expression where I is current of the order ofa few milliam'peres, Ris an effective resistance in ohms, hereinafterdefined, anda and b are constants. The exponent a determinesthe'curvature of-the exponentialv resistance curve of any onesemiconductor rectifier at one temperature. It is variations of theseconstants, a and b, after manufacture that is particularly troublesome.In my co-pending application: entitled Testing Semi-ConductorRectifiers, Serial.l;\l9.-336 ,893, filed February 13,1953, issued onApril24, 1956, as'U. Sglatent No. 2,743,420, I describe in detail howrectifiers may be tested and grouped ac- Other objects of my inventionand its true scope will appear as the following description of oneembodiment 2,874,448 Patented *Feb. 24, 195? ice 2 of my inventionproceeds. In the accompanying drawmgs:

Fig. 1 shows the effective forward resistance characteristic of typicalsemiconductor rectifiers,

Fig. 2 is a graph showing how the effective resistance of a rectifiervaries with current for different values of the constant b, and

Fig. 3is a graph showing how the effective resistance of a rectifiervaries with current for different values of the exponent a.

I have found that the resistance of a semiconductor rectifier varieswith the forward current through the rectifier. The typical effectiveresistance-current characteristic of commercial rectifiers arerepresented by the curves of Figs. 2 and3. Unfortunately, the currentdoes not vary linearly with resistance but varies exponentially andsuggests that the current-to-eifective resistance rela tionship may bedefined by the expression where I and R are, respectively, current andeffective resistance and where a and b are constants. If the value ofthe exponent a varies in use the curvature of the characteristic curvewill change as suggested in Fig. 3, in which case circuitry containingthe rectifier becomes unbalanced and decalibrated. It is particularlytroublesome having dissimilar a values cross as in Fig. 3.

Effective resistance is defined to be that linear or ohmic resistancewhich will pass the same average current during a specified voltagepulse as will pass a non-linear rectifier when subjected to the samevoltage pulse. It is like wise desirable that the constant b be stableduring life so that the static parameters of the rectifier will notchange and upset the circuitry in which it is incorporated.

It has been found that the conductivity of germanium may be changed whenthe crystals are bombarded by highenergy particles from a cyclotron.Bombarding the crystals appears to produce lattice dislocations andperhaps otherwise affect the electrical properties. I assumed, then,that the crystal structure and the position of the impurity atom mightbe changed with thermal-agitation, and I found this to be true when thecrystals were heated. Although the electrical changes were, small whenthe heatinduced energy of vibration was low, large changes occurredoverlong periods of time. The energy of vibration of an atom is increased asthe temperature of the crystal is increased. It then became clear thatexposing these germanium crystals to temperature fluctuations mightaccelerate the process of impurity migration to more stable locations. i

On the assumption that alternately heating and cooling a germaniumcrystal containing suitable impurity molecules would tend toshift themolecules to more stable positions, I measured the effective forwardresist; ance of a number of commercially available germanium rectifiers;Germanium rectifiers having effective forward resistances of the valuesindicated in Fig. 1 by curves 10 and 11, had 'an effective forwardresistance of about 330 ohms when received from the manufacturer. Asthese particular rectifiers alternately were chilled to 69" C." andheated to 0., forwardresistance continued to increase to 450 and 415'ohms, respectively, after four cycles of freezing and heating.Rectifiers with such widely variable forward resistances as these areobviously an: suited'for commercial uses where the operating tempera;tures may be extreme. Other rectifiers such as those hav: ing theforward resistances indicated by curves 12,13 and I4, on the other hand,decrease from an initial 440 "ohms to values below 400 ,ohms' as thecrystal was subjected alternately to extreme high and low temperatures.Surprisingly, as the cycling of the crystal through extreme temperaturescontinued it was found that the forward resistance changed less andless, percentage-wise. It

seems reasonable to assume from these results that alternate heating andcooling of the semiconductor-type rectifier will stabilize its effectiveforward resistance-current characteristics.

In determining the range of temperatures to which the crystal must besubjected for adequate stabilization, expermentation indicated that therange may be uite narrow or quite wide. It was found that by heating andcooling to temperatures, respectively, above and below the range oftemperatures to which the rectifiers would be subjected in use, thestabilization process could be accelerated. For some military purposesthese ranges may vary from the temperatures of the tropics to thesub-zero temperatures of the polar regions. Norrower temperatureoperating ranges are of course usual. The maximum temperature to which amanufactured semiconductor rectifier can be subjected appears to belimited only by the temperature to which the glass-to-metal seals andother mechanical parts of the rectifier may be carried without damage.The minimum temperature to which the crystals may be frozen likewise isdetermined by the mechanical characteristics ofthe rectifiers. It wasfound, for example, that with some commercial rectifiers the differencein coefiicient of expansion of the various parts may actually cause thecontact electrode such as a cat whisker to separate from or shift on thecrystal, or cause a fracture at the seals.

The number of heating-cooling cycles necessary to stabilize the forwardresistance characteristic of the rectifier is likewise individual toeach rectifier. The number of cycles in turn is dependent upon themagnitude of the temperature ranges employed. In the case of therectifiers graphed in Fig. 1, about 18 cycles, using temperatureextremes of 60 C. and +100 C., was considered satisfactory forsufficiently stabilizing the rectifiers for use in critically balancedcircuits. No appreciable drift in the forward resistance of rectifiersthus processed has been noted after several months of use. The number ofcycles of course may vary widely, the greater the number the greaterbeing the invariability of the forward resistance.

Good results have been obtained when the crystals are heated to +100 C.and heldat that temperature for 60 minutes and then refrigerated to 60C. for about 60 minutes each, the crystals being held at roomtemperature for about minutes between each heating and coolingexcursion. As suggested above, when commercially available germaniumrectifiers are thus treated for at least four cycles, the constants aand b in the resistancecurrent expression become su1ficiently stable formany circuit applications. The constants a and b fluctuate less and lessas the number of heating and cooling cycles increase.

Many modifications of my novel heating-cooling cycl ing may be madewithout departing from the scope of my invention. The time-temperatureconstant of each heat treatment, and the time-temperature constant ofeach cooling treatment should be sufficiently long, and the number ofheating and cooling cycles should be sufficiently large that they rendersubs antially invariable the constants a and b in the relationship whereI and R are the forward current and etfective resistance, respectively,of the rectifier in the operating temperature range of the rectifier,and a and b are said constants.

Although the temperature cycling described and hereinafter claimed veryprobably produces the etfects suggested relative to impurity locationstabilization, it is to be understood that the utility of the method andthe beneficial results which ensue may, in fact, be found in the futureto be the result of other changes; for example, changes at therectifyingjunction of the semiconductor, not clearly understood at the presentstate of the art.

I claim:

l. The method of treating a semiconductor-type rectifier havingdissimilar metals in contact, said method comprising heating saidrectifier to a temperature above the normal operating temperature of therectifier, and then cooling said rectifier to a temperature below thenormal operating temperature of the rectifier.

2. The method of treating a rectifier defined in claim 1 furthercomprising the step of holding the rectifier at sub stantially roomtemperature after the said heating excursion until the temperature ofthe rectifier substantially returns to room temperature.

3. The method of treating an assembled semiconduc tor-type rectifier tostabilize the effective forward resistance-current charactertistics ofthe rectifier comprising alternately heating and cooling said rectifierto temperatures outside the normal operating temperature range of saidrectifier.

4. The method of accelerating the aging and resultant stabilization ofthe forward resistance-current characteristics of semiconductorrectifiers, comprising heating said rectifiers to approximately degreescentigrade above zero and then cooling said rectifiers to approximately60 degrees below zero, and repeating the mentioned heating and coolingsteps a plurality of times.

5. In the method defined in claim 4, holding the rectifiers at each ofthe high and the low temperatures for approximately one hour.

6. In the method defined in claim 4, holding the rectifiers at roomtemperature for approximately one quarter hour between the heating andcooling excursions.

7. The method of stabilizing the resistance-current characteristic ofanassembled semiconductor-type rectifier said characteristic being definedby the relationship, I- =bR, where R is resistance, I is current and aand b are constants, said process comprising alternately heating andcooling said rectifier to uncritical temperature extremes of about +l00C. and about --60 C. maintaining the said rectifier for about one hourat the temperature extreme of each of the said heating and coolingexcursions, and continuing the heating and cooling excursions until saidconstant a reaches the desired degree of stabilization within apredetermined range of operating temperatures between the temperatureextremes of the aforesaid heating and cooling excursions;

8. The method of stabilizing the resistance-current characteristic of anassembled semiconductor-type rectifier, said characteristic beingdefined by the relationship, l- =bR, where R is resistance, I is currentand a and b are constants, said process comprising alternately heatingand cooling said rectifier between about +l00 .C. and about -60 C. forabout one hour for'each heating and cooling excursion, and continuingthe heatingand cooling excursions for at least four cycles.

9. The method of stabilizing the resistance-current characteristic of anassembled semiconductor-type rectifier, said characteristic beingdefined by the relationship, I" =bR, where R is resistance, I is currentand a and b are constants, said process comprising heating saidrectifier to about +100 C., then cooling'said rectifier to about -60 C.,and repeating the heating and cooling until said constants reach adesired degree of stabilization for a given temperature.

References Cited in the file of this patent UNITED STATES PATENTS2,395,259 Ellis et al. Feb. 19, .1946 2,464,066 Addink et al. Mar. 8,1949 2,602,763 Scaff et al. July 8, 1952

